Conventional metering systems for the application of liquid agricultural chemicals have long used positive displacement pumps, usually of the reciprocating variable-stroke type, driven by a ground engaging wheel or other means for driving the pump at a rate linearly proportional to vehicle speed. Such pumps are expensive, however, and their valving is particularly subject to abrasion from chemicals retained in liquid suspension rather than solution. Other disadvantages of using positive displacement pumps for such metering systems are described in U.S. Pat. Nos. 3,361,354 and 3,361,357, the disclosures of which are by reference incorporated herein.
Those patents essentially teach that a centrifugal pump, of the type having output pressure substantially proportional to the square of its speed, can be used for accurate metering with application means having a liquid discharge orifice, the output thereof, i.e. flow rate therethrough in amount per unit of time, being substantially proportional to the square root of the pressure of the liquid supplied thereto. In such a system the application rate, at a given vehicle speed, is set by setting discharge orifice pressure, i.e. upstream of the orifice, usually by adjusting a by-pass valve that returns a portion of pump delivery to a supply tank. Once so set the centrifugal pump metering system will maintain the application rate substantially constant at all other vehicle speeds, i.e. the output of or flow rate through the orifice will be substantially linearly proportional to vehicle ground speed.
The term application means is used herein to include both spray nozzles for surface application and ground penetrating blades or knives for subsurface application, it being understood that both have a discharge orifice, the rate of flow therethrough, in amount per unit of time, being substantially proportional to the square root of the pressure thereon. As taught in the aforesaid patents the pressure developed by certain types of centrifugal pumps varies substantially as the square of their speed (in RPM), so that when such a pump supplies liquid to such an orifice, flow therethrough is substantially linearly proportional to pump speed. U.S. Pat. Nos. 3,361,354 and 3,361,357 also disclose the provision of a calibrated dial liquid return or application means by-pass valve settable to produce the desired application rate by the use of a chart.
Most liquid agricultural chemicals are applied at the present time with large truck applicators having a centrifugal pump distribution system. An example of such truck applicators is the C-2950 Series made by the John Blue Co. of Huntsville, Alabama which are illustrated and described in Blue catalog "Liquid Fertilizer Equipment", Form No. 12T12 1174. The calibrated dial valve metering system is not, however, practical for such truck applicators. The pump usually is driven by a power take off from the truck engine, so that when the truck transmission gears are changed, the ratio of pump speed to vehicle speed is changed. Moreover, the transmission gear ratios, rear end ratios, tire size, and pump drive ratios differ from truck to truck depending on the make, model, size and equipment specified. This would require the preparation of an application rate chart for each particular truck and for each of its transmission gear ratios to be used during application. Further, the accurate determination of pump speed to vehicle speed for each different truck and each of its several transmission gear ratios is practically impossible. Additionally, the chart for the dial valve must be determined by actual flow test for each number and size of discharge orifices at each application rate it is anticipated may be used.
From the foregoing it will be seen that the preparation of application rate charts for truck applicators having calibrated dial valve centrifugal pump metering systems would be entirely too difficult and expensive to be practical.
Still further, the calibrated dial valve must either be installed in the truck cab, requiring extra hose and fittings and the cutting of holes in the cab for the large hose and pipe fittings, or else the valve must be installed outside of the cab requiring the driver to stop the truck and get out to adjust the valve. Neither of these alternatives is acceptable to a truck operator. Additionally, a calibrated dial valve is quite expensive because it must have extreme accuracy and be able to resist corrosion and abrasion from agricultural chemicals.
For the foregoing reasons, although most truck applicators employ a centrifugal pump distribution system, they lack metering accuracy because they are not equipped with a calibrated dial by-pass valve. Most such truck applicators, however, are provided with a return valve for sparging purposes, i.e. agitating and maintaining chemicals in suspension in the supply tank, and sometimes with a gauge for measuring pressure of the liquid supplied to the discharge orifices of the application means. Calculations can be made to determine the pressure with which liquid should be supplied to the discharge orifices at a given speed for a desired application rate. The actual application rate usually is inaccurate, however, because of the difficulty of setting the sparger or return valve manually to give the correct discharge orifice pressure for a given speed. Moreover, in practice an operator usually starts application at the wrong rate while trying to adjust the sparger valve to adjust discharge orifice pressure to achieve the correct preselected rate, almost always applying too much or too little. In fact, orifice pressure may be several hundred percent in error when the discharge valve is opened to commence application. While adjusting the sparger valve to obtain the correct orifice pressure serious over or under application of chemicals can occur. Such a situation is bad enough when applying fertilizer, but disastrous when applying herbicides.